1. Field of the Invention
The present invention relates to a heating device for heating an object, and more particularly to such a heating device adapted for use in a fixing apparatus for fixing an unfixed image in electrostatic printing, electrophotography, magnetography and the like, particularly such fixing apparatus as are designed for applying heat to an unfixed image to fix the same to an image bearing member.
2. Description of the Prior Art
In the field of heating elements there are already known ceramic materials having a positive temperature coefficient of electric resistance (hereinafter described as ceramics with PTC characteristics or PTC ceramics), and also known in the art are heating devices for image fixing utilizing such ceramic materials as the heat source.
Such PTC ceramics, capable of generating heat under a suitable voltage application, provide various advantages as the heat source in various heating devices, such advantages include a temperature self-control function without the external power control connected to a temperature detecting element, a limited variation in heat generation in response to the fluctuation in supplied voltage, and a long service life, etc. Also in the fixing apparatus for fixing a toner image they provide the advantages of rapid heating and shortened waiting time, in addition to those mentioned above.
However the PTC ceramics are associated with insufficient dimensional stability in the sintering and cooling steps, which becomes more marked as the molded products of PTC ceramics become larger in volume or in surface area. Also the PTC ceramics have difficulties in mechanical working such as scraping or grinding, so that the preparation of a relatively long roller or a relatively wide plate member with PTC ceramics for the fixing apparatus is practically quite difficult, giving rise to poor mass producibility and an elevated production cost. Furthermore the PTC ceramics tend to form cracks or local unevenness in quality in the sintering or cooling steps, particularly when they are large in volume or in surface area, thus becoming unable to provide uniform heat generation, over the surface of the heating element, or a desired amount of heat. In the fixing apparatus, such cracks or qualitative unevenness hinders the uniform heat application to the toner image or to the bearing member therefor, thus resulting in uneven or insufficient fixing.